Noncoalescence in Liquids of Similar Temperatures
Arzie W. Yeater III
Advisor: Dr. P.J. Cooney
While brewing coffee drops can be noticed skating along the surface
of the liquid instead of falling in. We made a movie to capture the events
and later progressed to a high speed video to investigate the physics of these
noncoalescing drops more closely. The drops formed, moved, and perished
in a consistent pattern, lasting about one second. We used Vernier’s
LoggerPro® software for video analysis and their LabPro® data logger for
temperature readings. The mechanism behind the noncoalescence involves
surface tension and vapor pressure gradient. The two properties are
oppositely dependant on temperature; vapor pressure exponentially builds
and surface tension linearly declines with rising temperature. Middle
temperatures around 60oC are the most likely place to form drops that do not
coalesce. Surface tension reflects intermolecular forces that hold drops
together, with small diameter drops held together more strongly. Smaller
drops are less likely to become part of the bulk liquid and vapor pressure
gradient creates a force that helps to keep the drops from the liquid. The
coffee evidently acts as a surfactant, forming a slight “scum” on the surface
increasing the barrier between drop and base coffee.